In the increasingly competitive steel industry, minimizing the energy consumption of a mill is one of the most important goals. Lowered energy consumption at the mills could mean lower prices for the consumers or higher profit margins for the mill owners or, sometimes, the survival of the business enterprise involved.
Because of the tremendous amount of energy consumed by a mill, savings of a small fraction of the energy used can result in significant monetary gains. An area in which such energy savings may be achieved is in the preheating of scrap metal prior to its insertion into a smelting furnace. By preheating the scrap metal, the smelting furnace requires less energy input to produce the same amount of molten metal.
Various devices and processes have been disclosed for recycling the energy remaining in the furnace-waste gases from a smelting furnace for use in preheating scrap metal. This is typically implemented by combusting the furnace-waste gases and then channeling the hot combustion gas, or heating gas, to a receptacle to preheat the scrap metal contained therein.
In a device for preheating steel scrap disclosed in German patent publication DE-PS 31 33 447, a receptacle that is open at the top and has a base cover which is selectively openable is inserted in a pit. A hood connected to a waste-gas line is fastened to an opening of the receptacle in a gas-tight manner. Hot waste gases are guided into the receptacle from the hood and through the receptacle from top to bottom, thus preheating the scrap contained therein. After the scrap is preheated, the waste gases are guided through an opening provided in the base cover.
In a further development of this concept, German patent publication DE-PS 33 07 400 discloses a preheating apparatus in which hot waste gases are guided axially through a waste gas line from top to bottom through a basket or cage. The basket has solid side walls open at the top and a permeable bottom that is selectively openable. A carriage, or car, is used to transport the basket to the preheating apparatus. The carriage is driven to a position under a raised hood connected to a waste-gas line, and the hood is then lowered until its rim contacts the rim of the basket. The permeable bottom of the basket rests on a mouthpiece located at the carriage and connected to the waste gas line.
Both of these prior art devices employ only one basket connected to a waste-gas line, and the waste gases always flow through the basket from top to bottom. In addition, the hot gas supply to the preheating apparatus is interrupted during each changeover as the basket containing the preheated steel scrap is removed from the preheating apparatus so as to transfer the preheated scrap into the smelting furnace.
An arrangement for preheating scrap having a rotatable platform for two or more scrap baskets is disclosed in German patent publication DE OS 32 43 128. A basket containing preheated scrap is swiveled away from a waste gas line while a new basket is swiveled toward the waste gas line. Gas lines provided with valves or valve systems lead to the scrap baskets. The hot gas can be kept away from or guided away from or guided past the scrap basket or scrap baskets during a changeover by means of the valves or valve systems. The rotating platform and the valves of a duct system for the hot air or hot gas are so constructed that a first basket and a second basket can be heated simultaneously while a third basket location facilitates a changeover. This reference, however, does not disclose the construction of the baskets although it appears, based on the disclosed construction of the installation, that conventional baskets provided with base covers are intended to be used.
A disadvantage of the device disclosed by German patent publication DE-OS 32 43 128 is that it is necessary to guide the gas away from or past the preheating station when changing the baskets.
Another disadvantage of the scrap preheating arrangements disclosed by the above-mentioned prior art is that the hot waste gas is guided from top to bottom of a basket so that some constituents, notably small particles, are also melted along with the steel scrap. These constituents are entrained by the gas flowing downward through the burden column from the top and settle on or near the cooler base of the basket in the form of so-called skull.